Polyvinyl alcohol (hereinafter, also referred to as PVA), which is excellent in transparency, oil resistance, chemical resistance, and gas (e.g. oxygen) barrier properties, has been widely used as a packaging material. In recent years, a packaging material for foods, drugs, industrial chemicals, agrochemicals, or the like whose characteristics are largely affected by oxidation degradation has been required to have higher oxygen barrier properties.
In cases where a PVA resin is used for various packaging materials, commonly, the PVA resin is first dissolved in water to give a PVA aqueous solution, and the PVA aqueous solution is cast to be formed into a film or applied to a base material. However, since the PVA aqueous solution has a high surface tension, the PVA aqueous solution is easily foamable and is likely to be formed into a film before defoamation of the applied solution is sufficiently performed. The gas barrier properties of such a film are significantly lowered due to the bubbles therein.
For producing a PVA cheese cloth or a PVA heat-insulation curtain to be used in a vinyl house, a PVA film needs to be stretched. If the PVA film contains bubbles or contaminants therein, the PVA film upon stretching may be torn from the portion where bubbles or contaminants are contained to be ruptured, which results in a great cost loss. Even if the film is not ruptured upon stretching thereof, defectives in the film is widened by stretching to provide an improper appearance.
A PVA film, especially a PVA film with a high degree of saponification is used for packaging textile products. A PVA film even with a high degree of saponification absorbs or releases moisture in accordance with a humidity change therearound. Accordingly, if the package is left for a long period of time, the plasticizer in the film may bleed out to the film surface, and the bled plasticizer may migrate to the packaged product or be scattered together with the absorbed or released moisture.
To solve such problems, Patent Literature 1 discloses that the use of a resin composition containing a water-soluble PVA resin having a 1,2-diol structural unit and a water-swelling layered inorganic compound enables preparation of an aqueous coating liquid having low foaming and high defoaming properties.
Patent Literature 2 discloses a method for producing a PVA film including the steps of passing a film-forming material containing PVA through a filter and forming the film-forming material into a film. Patent Literature 2 teaches that a PVA film to be stretched can be produced by filling, when a filter is changed, a new filter with water, an aqueous solution, a liquid soluble in water, or a liquid mainly containing a water-soluble liquid.
Patent Literature 3 discloses that the use of a compound having a specific hydroxy group and a specific carboxyl group enables production of a PVA film in which a plasticizer contained in the film does not bleed out to the film surface and the flexibility can be maintained even after standing under severe conditions such as low temperature and low humidity conditions or high temperature and high humidity conditions.